Google announced today that it has signed a definitive agreement to acquire Postini, a leader in on-demand communications security and compliance solutions.

Postini's services -- which include message security, archiving, encryption, and policy enforcement -- can be used to protect a company's email, instant messaging, and other web-based communications. Under the terms of the agreement, Google will acquire Postini for $625 million in cash, subject to working capital and other adjustments, and Postini will become a wholly-owned subsidiary of Google. The agreement is subject to customary closing conditions and is expected to close by the end of the third quarter 2007.